1. Field of the Invention
The present invention generally relates to heating ovens; for example, electric or microwave heating ovens. The invention particularly relates to microwave ovens having mechanical safety switches linked to respond to the open-close action of their front doors, which mechanical switches also serve as thermally responsive safety switches. Thus, the switches both guard against operation of the microwave when the door is open, and against operation when the microwave is overheated.
2. Description of the Related Art
In the conventional microwave heating oven, a box which constitutes a heating chamber, is further enclosed in an external case. Because of such construction, it is difficult to make such a heating oven at a low cost, or to make such a heating oven easy in manufacture, or to make such a heating oven compact. In order to solve such problems, a heating oven in which the box of the heating chamber serving as the external case has been tried. For example, a heating oven wherein the control box containing electric parts is disposed at the side of the heating chamber, is shown in patent application Sho No. 59-105036, or a heating oven wherein the control box disposed below the heating chamber, is shown in patent application Sho No. 59-119319.
FIG. 1 shows one of such conventional microwave heating ovens. In FIG. 1, the control box 2 containing electric parts is disposed below the heating chamber 1. The front door 3 is disposed at the front opening 1a of such heating chamber 1. The front door 3 has a hook 3a for locking the front door 3. And the safety switch 7 is disposed at the position involving such hook 3a on the side wall of the heating chamber.
FIG. 2 shows a control circuit of such a conventional microwave heating oven. In FIG. 2, the control circuit comprises, a magnetron 31, a high voltage transformer 32, a high voltage capacitor 33, a diode 34, a cooling fan motor 4, a timer control motor 5, and so on. In the safety standard, for example UL standard, at least two safety switches 7 and 8, responsive to the open-close action of the front door 3 must be included in the design of the microwave. Further, a short switch 10 also must be disposed on such electric circuit, to melt down the fuse 9 in case switches 7 and 8 become stuck closed when front door 3 is open. Furthermore, the conventional microwave heating oven has a thermal switch 11 which is, for example, a fuse switch or a bimetal switch, so as to break off the electric circuit for making microwave 30 and the cooling fan motor 4 in case of abnormal high temperature or fire trouble of food. In practical design of the microwave heating oven, the position of the thermal switch is determined after designing of a passage of the exhaust gas from the cooked food by trial and error of the experiment.
As improvement toward simplicity of the configuration of the heating oven proceeds, however, the safety switch responding to the open-close action of the front door must be disposed at the position to be showered by the microwave, which is shown in FIG. 1. So that, such a heating oven has the following problems. First, shower of microwave destroys the safety switch, second at receiving such microwave shower, wires of control circuit radiate electric noise which a badly affect the television set or the radio.
Further, in a microwave heating oven such as shown in patent application Sho No. 59-119319, the walls of the heating chamber serves as the external case, and so the heat is conveyed toward external part easily. Therefore, there is a strong demand for the quick responding thermal switch which can break down the electric circuit in a short time.